A friend of mine was grey by the time he was 21. He only lived into his forties, but he never seemed to age!
I know a lady who is only a couple of years away from 80, does not dye her hair, and has only a couple of gray hairs.
So I suppose Stallone's <<might>> be natural.....
Istarted to go "ash blonde" around 22,now 61 with a reasonable head of hair. Always possible to dye it if it causes ambarassment. Seems more unusual than going thin on top,don't know why that is
Baldness is a dominant gene (Bb or bb, each gained from one parent) so full hair is recessive for men (BB).
Going grey early is rare but a specific condition called canities, whereas usually by the time men start in the 40s there's already some loss, so technically difficult for a full head of hair to go grey as baldness begins earlier on average.